ROOT-9999, with system llvm/clang

## About how to build

Patched llvm/clang are needed. Patches in cling repo are outdated. Recently ROOT updates its llvm/clang trunk version, and [these patches](https://github.com/ustcscgy/etc-portage/tree/master/patches/sys-devel) are generated and maintained to catch up newer version of llvm/clang.

These patches need minor modify if you use llvm/clang from gentoo portage, because I build llvm/clang from my local overlay and they are built by cmake.

If there is a better way to do this, please discuss about it.